1. Field of the Invention
The present invention pertains to a pressure fluid motor such as a rotary vane type motor which is provided with a flow sensing speed control device for throttling the flow of pressure fluid in the motive fluid exhaust passage.
2. Background
In the art of pressure fluid motors there are many applications wherein the motor is subject to a sudden reduction in its driven load or the motor may, while being supplied with motive fluid, be decoupled from the load either intentionally or inadvertently. In such instances it is desirable to be able to limit motor speed to prevent damage to the motor and/or the load which would result from an overspeed condition. Several inventions have been developed in the art of air and gas motor governors or speed control mechanisms including, for example, mechanisms which utilize centrifugal flyweights or the so called flyball governor principle. Although these devices are widely used, they are mechanically complicated and expensive and are not easily adapted to some types of fluid motors. Other types of controls for pressure fluid motors are adapted to sense a pressure differential across an orifice or restriction in the fluid flow path. These types of controls are subject to unstable operating characteristics and are susceptible to malfunction due to contaminants in the working fluid.
Certain types of air and gas motors, for example, also are adapted for reversible operation wherein dual motive fluid inlet ports are provided for introducing pressure gas into the working chambers of the motor to selectively vary the direction of rotation of the motor. In reversible rotary vane type fluid motors, in particular, the alternate fluid inlet port can be used as a secondary exhaust port when the motor is being rotated in one direction, and the port function can be reversed to effect reversal in the direction of rotation of the motor. In motors having dual inlet ports wherein one port functions as an auxiliary exhaust port, depending on the direction of rotation, it has been determined that the flow of fluid exiting the motor through the auxiliary exhaust port may be sensed to effect control of motor speed such as by throttling the exhaust fluid flow through the main or primary exhaust port. This method of motor speed control has been discovered to be effective and relatively uncomplicated and does not preclude the use of the motor as a bi-directional or reversible motor.